This invention relates to an end-burning grain of a solid propellant or a solid gas-generating composition for any other purpose, in which grain is embedded a metal heat conductor in the form of wire with the intention of increasing mass burning rate of the grain by increasing the burning surface area.
In solid propellant rocket motors and pyrotechnic gas generators for various purposes, end-burning grains that burn in the cigarette fashion from one end are used where it is desired to continue stable generation of combustion gases for a relatively long period of time. For a given composition the duration of gas generation is approximately proportional to the grain length, and the rate of gas generation or mass burning rate is approximately proportional to the burning surface area given by the area of the exposed end surface at which the grain is ignited.
With regard to solid propellant rocket motors of the end-burning type by way of example, the length of the motors and hence the length of the propellant grains can be determined with enough freedom to maintain the thrust for a required period of time, but in most cases great restrictions are placed on enlargement of the motor diameter and hence the burning surface are of the propellant grains with a view to increasing the thrust, which depends fundamentally on the mass burning rate of the propellant grains. If a considerable increase in the thrust is wished without increasing the motor diameter correspondingly, the wish must be fulfilled by employing a propellant inherently high in burning rate or by modifying a conventional propellant composition so as to attain a sufficiently increased burning rate. However, there is a limit to the normal burning rate of available solid propellants, and much difficulties and restrictions are posed on practical modifications of conventional propellant compositions or development of a new propellant composition.
As a solution of the above described problem, U.S. Pat. No. 3,116,692 teaches that the effective burning surface area and hence the mass burning rate of a solid propellant end-burning grain can greatly be increased by embedding a metal heat conductor in the grain and recommends to use a continuous wire as the metal heat conductor. As the most suitable way of putting this method into practice, the wire is embedded in the propellant grain normal to the exposed end surface of the grain to axially extend in the direction of flame propagation over a sufficient distance from the end surface, conveniently over the full length of the grain. The favorable effect of the embedded wire on the burning rate of the grain originates in a considerably higher heat conductivity or thermal diffusivity of the wire than the propellant material. As the grain is ignited at the exposed end surface and the propellant burns in the cigarette fashion an end portion of the wire is strongly heated by the hot combustion gas, and there occurs rapid and continuous transfer of a considerable amount of heat through the wire from the flame zone to the unburned portion of the propellant. Then burning proceeds rapidly along the wire probably by reason of accelerated decomposition of the oxidant in the unburned propellant, so that the burning surface begins to dent forwardly in a region adjacent to the wire to give a conical surface with the wire at its apex. The denting continues until the cone encompasses the entire width of the grain and brings about a great increase in the burning surface area. Owing to the increased rate of burning along the wire and the increased burning surface area, the grain burns with a greatly increased rate of gas generation. If desired, a plurality of suitably spaced wires may be embedded parallel to each other in a single grain.
Experimental results show an interesting fact that the effect of the embedded wire on the burning rate depends significantly on the thickness of the wire and becomes maximal when the thickness or diameter of the wire is about 0.1 mm. This fact may be explained as follows. Considering that the increase of the burning rate attained by the presence of the wire is primarily attributed to a rise in the temperature of the unburned propellant adjacent to the wire and that the wire is surrounded by the mass of the low temperature propellant, the effect of the wire will become appreciable only when the heat capacity of the wire, which is determined by the wire thickness, is large enough to conduct a fair amount of heat from the flame zone to the unburned propellant. On the other hand, the degree of heating of the wire by the hot combustion gas will lower as the heat capacity of the wire becomes larger, so that the heat-conducting effect is gradually negated as the wire diameter increases beyond about 0.1 mm.
In practical applications of the above described technique according to U.S. Pat. No. 3,116,692, however, it is quite difficult and almost impossible to attain a maximal extent of increase in the burning rate by using a wire having an optimum diameter of about 0.1 mm because of breaking or significant bending of such a thin wire during embedding of the wire in a solid end-burning grain accomplished at the stage of casting the grain, for example, by using a relatively high viscosity slurry of the propellant materials. For rocket motors and other types of gas-generating devices, the experience has taught that the wire to be embedded longitudinally in an end-burning grain needs to be at least about 0.2 mm in diameter in order to resist mechanical stresses exerted thereon during the grain-forming and wire-embedding operations.